1. Field of the Invention
This invention relates to optical beam test systems, and more specifically to an interferometer having a micromirror for measuring the quality of an optical beam wavefront.
2. Description of the Related Art
Several systems exist for extracting a reference beam from a beam of a source under test. Such reference beams have been found useful in applications such as interferometry. Some systems use a pinhole, i.e. a very small opening, to generate a reference beam from the beam coming from the source to be tested. Generating a reference beam from the original source beam helps to provide a reference beam which has the same wavelength and a constant phase relationship to the original source beam. Frequently, a pinhole is used to provide a reference beam which is relatively free from the effects of aberration present in the source beam. It is well known that when a sufficiently small pinhole is placed in the path of an aberrated beam, a relatively clean beam is produced because most of the energy present due to the aberration is not passed.
In some existing interferometer systems, a beam expander has been used to remove aberration from the beam used as a reference. An existing interferometer system is described in xe2x80x9cA Phase Measuring Radial Shear Interferometer for Measuring the Wavefronts of Compact Disc Laser Pickupsxe2x80x9d, B. E. Truax, Proceedings of SPIExe2x80x94The International Society for Optical Engineering, Vol. 661 (1986), 74 (xe2x80x9cTruaxxe2x80x9d). In the system described therein, the interferometer is placed at the output of a laser beam source. A beamsplitter splits the source beam into a test beam and a reference beam. An aperture is used in conjunction with a beam expander to remove aberration from the beam to be used for reference.
Such aperture/beam expander systems generally accept a collimated beam input and produce a collimated output. In such systems, the beam is passed through an aperture to filter out aberration energy which is proportionally greater away from the center of the beam. The resulting filtered beam, now narrower, is then expanded, in order to restore it to the width of the source beam.
As stated, such systems generally accept a collimated beam input. Since the beam is collimated, its energy is not as concentrated in the center as in a focused beam. In a 10% aperturexe2x80x9410X beam expander system, the aperture has one tenth the diameter of the beam. Thus, the area of the aperture is approximately one hundredth of the area of the original beam. When such a 10% aperture system is used as typically with a collimated beam, substantially all of the energy of the beam does not pass the aperture. This great loss in energy places a lower limit on the power level of the sources which can be tested in such systems.
In the system described in Truax, the test beam and the reference beam returning from the beam expander are recombined in the beamsplitter and guided by a set of lenses and a mirror to form an interference pattern on the pickup of a video camera. The interference pattern is analyzed to generate data representing the departure of the source beam from that which produces an ideal wavefront.
A disadvantage of the system described in Truax is that the interferometer must be reconfigured to a different setup from that used for testing in order to check and correct the alignment and placement of the system elements and the source. For example, in the system described in Truax, a mirror is slid into place to block light from striking the video camera from the normal direction used in test. The beam from the source is focused onto the center of a surface where a pinhole lies. A portion of the focused beam is reflected back from the surface and guided through alignment lenses to the sliding mirror where it is reflected into the video camera.
An LED is used to illuminate the pinhole from the reverse direction as occurs during the test. Elements of the system are then adjusted and aligned while the pinhole is illuminated from the rear so that the backlit image of the pinhole overlays the image of the focused spot produced by the source to be tested. When alignment is completed, the sliding mirror is slid out of the path of the beam, and the interferometer is reconfigured to the test setup.
Changing the test setup during the alignment operation has the disadvantages of increased system complexity and the inability to permit the alignment to be checked while the system is configured for testing.
It is, therefore, an object of the present invention to provide an optical filter which provides a first beam relatively free of aberration while simultaneously providing a second beam which can be used for alignment observation purposes.
A further object of this invention is to provide an interferometer which has a micromirror for generating a reference beam which is relatively free of aberration present in the source beam.
Another object of the present invention is to provide an interferometer, the setup of which need not be altered during the alignment operation.
These and other objects of the invention are provided by an interferometer having a micromirror for generating a reference beam which is relatively free from the effects of aberration in a source beam.
The interferometer is used to create and detect an interference pattern for output to an interference pattern analyzer. The interferometer according to the present invention accepts a source beam input from a source under test. The present interferometer is provided with a beamsplitter for splitting the source beam into a test beam and a reference beam. The interferometer has a mirror disposed in the path of the test beam for reflecting the test beam back toward the beamsplitter. This mirror can be movable longitudinally with respect to the test beam for varying the phase of the test beam in relation to the phase of the reference beam.
A micromirror is placed in the path of the reference beam for reflecting a portion of the reference beam back to the beamsplitter. Focusing means, such as a lens, in the path of the reference beam between the beamsplitter and the micromirror is used to focus the reference beam onto the micromirror. The micromirror has a reflector of lateral dimension which does not exceed the approximate lateral dimension of the central lobe of the spatial intensity distribution of the reference beam focused thereon by the focusing means. The lateral dimension of the micromirror is preferably about one third of the lateral dimension of the central lobe of the spatial intensity distribution of the focused reference beam. The interferometer preferably includes an alignment detector positioned behind the micromirror.
The micromirror of the present invention, having a reflective area smaller than the central lobe of the focused reference beam, also serves as a spatial filter for reducing the effects of aberration in a beam.
The filter includes a reflector having a lateral dimension which does not exceed the approximate lateral dimension of the central lobe of the beam focused upon the reflector. The lateral dimension of the reflector preferably does not exceed approximately one third the lateral dimension of the central lobe of the focused beam.
The present invention also provides a method for filtering a beam. The beam is focused upon a reflective-transmissive surface. A central portion of the beam is reflected while another portion lying outside the central portion is transmitted. The central reflected portion does not exceed the approximate lateral dimension of the central lobe of the spatial intensity distribution of the focused beam.
The dimension of the portion reflected is preferably about one third of the lateral dimension of the central lobe of the beam""s spatial intensity distribution. It will be appreciated by those skilled in the art that the accuracy and intensity of the reflected beam are influenced by the dimension of the portion selected to be reflected. Selecting a smaller portion for reflection will yield a more accurate, i.e. less aberrated reference beam, while selecting a larger portion for reflection will yield a reference beam of greater intensity.
The micromirror functions as, and provides, a spatial filter which simultaneously provides a reflected beam portion known as a reference beam for interfering with a portion of the unfiltered incoming test beam, and a transmitted beam portion to be received by an alignment observation system.
A method for filtering a beam permits a portion of the filtered beam to be received by an alignment observation system. The method permits the pointing and alignment of a wavefront measurement system, e.g. an interferometer, to be monitored without a change in the configuration of the wavefront measurement system.